Moscow can hit back at "any British military installations and equipment in or outside Ukraine" if Ukraine strikes on Russian territory with U.K.-made weapons, the Russian Foreign Ministry claimed on May 6.
British Ambassador to Moscow Nigel Casey was summoned to Russia's Foreign Ministry for a formal protest after U.K. Foreign Secretary David Cameron said that Kyiv had a right to use the weapons provided by London to strike targets inside Russia.
"Ukraine has that right. Just as Russia is striking inside Ukraine, you can quite understand why Ukraine feels the need to make sure it's defending itself," he said.
The Russian ministry claimed that Cameron "had de facto recognized his country as a part of the conflict," calling his remarks a "serious escalation."
"Casey was warned that in response to Ukrainian attacks on Russian territory with British weapons, any British military facilities and equipment on the territory of Ukraine and abroad could be targeted," the ministry's statement read.
Russia has repeatedly threatened to use nuclear weapons in response to Western support for Ukraine, but the threats have so far failed to materialize.
Ukraine has continued to press its Western allies for longer-range weapons, but partners have hesitated about delivering arms that could potentially be used to strike within Russian territory.
President Volodymyr Zelensky has said that Kyiv will not use weapons supplied by foreign partners to hit targets outside of the country's borders. Such restrictions do not apply to domestically produced arms, some of which are reportedly capable of striking deep into Russia.
Some of Ukraine's allies have sent weapons to Kyiv with no restrictions on strikes inside Russia, according to Latvian Foreign Minister Baiba Braze.
Ukraine has also reportedly used U.K.-supplied Storm Shadow missiles to target Russian military targets Crimea, a sovereign Ukrainian territory illegally annexed by Russia.